Supply-switching system

Abstract

A system for providing a first voltage generated by a main supply and a second voltage generated by a battery to an integrated circuit (IC) includes supply-selection, control logic and switching circuits. The supply-selection circuit includes first, second, and third transistors. The switching circuit includes fourth and fifth transistors that supply the first and second voltages to the IC when switched on. The supply-selection circuit selects and provides the higher of the first and second voltages to body terminals of the fourth and fifth transistors for maintaining required body-bias voltage conditions. The control logic circuit generates a first control signal as long as the first voltage is within a predetermined range for keeping the fourth transistor switched on and a second control signal when the first voltage is not within the predetermined range for switching on the fifth transistor to supply the second voltage.

The invention claimed is: 
     
       1. A supply-selection circuit for selecting a higher of a first voltage generated by a main supply and a second voltage generated by a battery, comprising:
 a first transistor having a drain terminal connected to body and gate terminals thereof, and a source terminal connected to the main supply for receiving the first voltage, wherein the drain terminal thereof outputs the first voltage when the first voltage is higher than the second voltage; 
 a second transistor having a drain terminal connected to body and gate terminals thereof, and a source terminal connected to the main supply for receiving the first voltage; and 
 a third transistor having a drain terminal connected to the drain terminal of the first transistor, a body terminal connected to the drain terminal thereof, a gate terminal connected to the body terminal of the second transistor, and a source terminal connected to the battery for receiving the second voltage, wherein the drain terminal thereof outputs the second voltage when the second voltage is higher than the first voltage. 
 
     
     
       2. The supply-selection circuit of  claim 1 , further comprising a fixed-current source connected between the drain terminal of the second transistor and ground for regulating a drain current of the second transistor. 
     
     
       3. The supply-selection circuit of  claim 1 , wherein the first, second, and third transistors each comprise of a p-channel metal-oxide semiconductor (PMOS) transistor.